The invention relates to a micromotor with a stator comprising a sleeve with an excitation winding and an enveloping flux return structure of spaced ferromagnetic rings, and particularly a micromotor combined with a pump portion and forming an impeller pump.
In the field of medical technique, blood pumps are needed which are introduced into the body of a patient and placed in an artery or the heart to support the heart function. An intracardiac blood pump provided for operation in the heart is described in DE 198 21 307. The blood pump comprises a drive portion wilth an electric micromotor and a pump portion. It has a maximum outer diameter of about 8 mm and a rigid length of less than 35 mm. The small-scale blood pump having the required flow rate requires a high rotational pump speed of at least 30,000 revolutions per minute, typically of 60,000 revolutions per minute.
A micromotor suitable for the operation of a blood pump is described in WO 98/44619. This micromotor has a stator and a permanently magnetized rotor. The stator includes an excitation winding surrounded by an enveloping flux return structure. The enveloping flux return structure bundles the magnetic field lines generated by the excitation winding and concentrates them so that only little magnetic flux is lost by stray fields. The enveloping flux return structure consists of pieces of sheet metal which are electrically insulated with respect to each other and assembled to a tubular stack.
The wish to introduce pumps with a catheter into the vascular system of a patient by punction without being forced to operatively open the patient body results in the need of ever smaller pumps and motors the diameters of which are in the range of 4 mm. With such small dimensions, there is only very little room available for the enveloping flux return structure in the case of an electric micromotor. Here, the enveloping flux return structure can only have a wall thickness of a few tenths of a millimeter. An enveloping flux return structure of such a small wall thickness cannot be assembled from pieces of sheet metal. On the other hand, an enveloping flux return structure is useful in order to avoid a loss of energy by magnetic field leakage.
It is the object of the invention to provide a micromotor with high efficiency and small radial dimensions. This object is solved, according to the invention, with the features indicated in claim 1. Accordingly, the enveloping flux return structure consists of an integral body wherein adjacent rings are connected by at least one bridge. This means that the enveloping flux return structure is made of an integral tube, and the rings are not completely electrically insulated with respect to each other as is the case with sheet metal pieces. They are rather interconnected, on the one hand, and spaced, on the other hand, by the bridges. It is required to divide the enveloping flux return structure into rings to limit the formation of eddy currents. Conventionally, the rings are arranged separately from each other and completely electrically separated from each other by insulating layers. In the case of the micromotor according to the invention, the rings are not completely separated from each other, the bridges, however, which are only arranged at some locations of the periphery, do not lead to the formation of substantial eddy currents. Consequently, the entire enveloping flux return structure can be made of a continuous body. The enveloping flux return structure can have a wall thickness of 0.2-0.4 mm, i.e., an extremely small wall thickness, the bridges interconnecting the rings providing the required cohesion and the necessary strength. Due to the small wall thickness of the enveloping flux return structure, it may occur that the enveloping flux return structure is operated in magnetic saturation without being able to receive the entire magnetic flux. The micromotor according to the invention has a rotational speed of at least 30,000 revolutions per minute and typically of 60,000 revolutions per minute. The enveloping flux return structure has the required structural strength required for the assembly as well as for the operation of the micromotor. If thin lamellae were glued together, such a structural strength could not be achieved.
According to a preferred development of the invention, the bridges of the enveloping flux return structure form a longitudinal continuous web. This means that the bridges are oriented relative to each other. This orientation can be effected along a line parallel to the motor axis or preferably along a helical line. Preferably, there are at least two webs which are distributed around the circumference.
According to a preferred development of the invention, the enveloping flux return structure integrally passes into a pump housing. The enveloping flux return structure divided into rings in the region of the micromotor is continuous in the region of the pump housing. Because of the integrity of pump housing and enveloping flux return structure, a particularly exact centering of the pump housing relative to the micromotor is achieved. The assembly and the demands of precision made thereon are simplified.
Further, the invention relates to a method of manufacturing a micromotor. In this method, the enveloping flux return structure of the micromotor is manufactured of a ferromagnetic tube by laser cutting, in the course of which peripheral slots interrupted by bridges are generated. Hereinafter, an embodiment of the invention is explained in detail with respect to the drawings.